Asymmetries and symmetries in the number of thoracic dorsal root ganglion cells.

The right/left ratio for the number of neurons in individual pairs as well as series of pairs of normal rat and guinea pig thoracic dorsal root ganglia have been investigated. After perfusion of the animals with aldehyde fixatives, the 13 pairs of thoracic ganglia were embedded in resin, serially sectioned, and stained with azure methylene blue. Counts of neuronal nucleoli (with correction for possible split nucleoli) were used as an indicator of the number of neurons in each ganglion. In individual pairs the right/left difference varied between 0 and 47% in the rat and 0 and 26% in the guinea pig. There appeared to be no particular level with a preference for large right/left differences. However, when the total number of neurons from the 13 ganglia on one side were compared with the corresponding figure from the other side, the right/left differences were reduced to less than 3% in the rat and less than 2% in the guinea pig. These results show that the organization of the thoracic spinal nerve unit is markedly asymmetrical at the segmental level, but strictly symmetrical in terms of the total number of dorsal root ganglion cells. The marked asymmetry in individual ganglion pairs necessitates caution when evaluating the effect of unilateral experimental manipulations on the number of neurons in thoracic spinal ganglia.